1. Field of the Invention
The present invention relates to an image formation device and an image formation method.
2. Description of the Related Art
Conventionally, an image formation device which forms an image by generating and fixing a toner image on a sheet has been known. More specifically, the image formation device includes a fixing unit which executes a fixing process for fixing the toner image on the sheet by heating and pressing the generated toner image. Here, it should be noted that the fixing unit generally consists of a pair of rollers, whereby the sheet which passes the pair of rollers is heated by a heater provided inside or outside at least one of rollers constituting the relevant pair. Incidentally, in the fixing unit, a temperature which is necessary to execute the fixing process is maintained by the heater which executes heating to supply an amount of heat absorbed by the sheet passing the pair of rollers.
Incidentally, the image formation device is required to execute the fixing process in regard to various different kinds of sheets whose sheet attributes such as a material, a thickness and the like vary from one kind of sheet to another. Moreover, even in case of executing the fixing process to the various kinds of sheets, the image formation device is required to execute an image formation process, including the fixing process, at constant sheet transportation speed.
However, in the case where the image formation device executes the fixing process in regard to the sheets at the constant sheet transportation speed, the amount of heat to be applied from the pair of rollers to the sheets is constant. For this reason, it is difficult to make fixability of the toner images, which have been respectively formed on various kinds of sheets of which the thickness and material are different from others, the same.
Consequently, a method of providing plural fixing units has been proposed (e.g., Japanese Patent Application Laid-Open No. 2005-292651). In this method, the number of sheets on which images are respectively formed per unit of time can be made constant and also degradation of fixability of the formed images on the respective sheets can be prevented, irrespective of kinds of sheets.
An image formation device disclosed in Japanese Patent Application Laid-Open No. 2005-292651 includes, as sheet transportation paths, a main transportation path through which the sheet passes plural fixing units and a roundabout transportation path through which the sheet passes only one fixing unit. More specifically, in the relevant image formation device, since the main transportation path and the roundabout transportation path are provided, the number of sheets on which the images are respectively formed per unit of time can be made constant and also the degradation of the fixability of the formed images on the respective sheets can be prevented, irrespective of the kinds of sheets.
However, in Japanese Patent Application Laid-Open No. 2005-292651, if at least one of the plural fixing units cannot be used due to a breakdown or the like, there is no way to cope with such a case.
In any case, if any one of the plural fixing units cannot be used due to the breakdown or the like, it is undesirable to completely stop an image formation process including a fixing process. That is, it is desirable to execute the image formation process as much as possible so as to increase the number of sheets on which images are respectively formed per unit of time.